Ophthalmic lenses are produced by several known processes, including spin casting, precision lathing, and molding. In spin casting processes a suitable ophthalmic lens composition is concurrently rotated and polymerized in a mold, as described generally by Nandu et al. in U.S. Pat. No. 5,260,001. Precision lathing is performed by lathing a piece of clear polymeric material into the shape of a lens, and polishing the lens thus formed. Precision lathing is described generally by Le Boeuf et al. in U.S. Pat. No. 4,054,624. Molding operations are carried out by polymerizing a suitable material in a preformed mold, as described generally by Larsen in U.S. Pat. No. 4,640,489.
Various compositions are used to manufacture ophthalmic lenses. The composition chosen for a particular application depends upon the physical and optical properties of the lens that one is attempting to obtain, as well as the type of process used to manufacture the lens and the particular processing conditions associated with such process. A particularly popular type of ophthalmic lens is the soft contact lens, which is made from compositions that are hydrophilic, and thus absorb water. Exemplary hydrophilic lenses are based upon polymers and copolymers of 2-hydroxyethylmethacrylate.
A soft contact lens that is gaining wide acceptance is the extended wear lens, which people can keep on the cornea surface overnight, and for periods of time exceeding seven days. Extended wear lenses must meet special physical criteria because the cornea relies for oxygen upon diffusion from the ambient air, and a contact lens is a physical barrier to such diffusion. To overcome this physical barrier, manufacturers have developed special oxygen permeable compositions for extended wear lenses. Such oxygen permeable compositions generally contain elements or compounds that attract oxygen, such as silicon and fluorine. A variety of siloxane-containing polymers having high oxygen permeability are described, for example, in U.S. Pat. Nos. 3,228,741, 3,341,490, 3,996,187, and 3,996,189.
Ophthalmic lenses are often produced from compositions that include a volatile component such as a solvent or diluent. For example, many ophthalmic lenses are manufactured from polymerizable compositions in which the monomeric component(s) are solubilized in an appropriate solvent. Many ophthalmic lenses are manufactured with carrier volatile components, which are extracted from the lenses after polymerization and replaced with water. Whenever volatile components are included in ophthalmic lens compositions, extreme care must be taken to prevent evaporation of the volatile component(s), which can alter the ratio of ingredients in the composition. To minimize the risk of evaporation, exposure to ambient gasses is minimized, and excessive turbulence is also minimized.
Ophthalmic lenses must meet very demanding standards. As previously noted, extended wear lenses must be sufficiently hydrophilic and permeable to oxygen. In addition, ophthalmic lenses must be strong enough to withstand tearing, and they must meet demanding dimensional requirements to achieve the prescribed optical correction, and to match the corneal dimensions of the wearer. Moreover, the lens must be extremely thin, to facilitate oxygen permeability, and to enhance the comfort to the wearer. The lens must also be clear and without distortion, to provide pleasing and precise optical correction. Processing conditions that can meet the stringent lens standards are very demanding, and manufacturers establish rigorous quality control programs, and continuously evaluate and revise their operating procedures, to minimize the incidences of defective lenses.
A major concern in any quality control program is contamination, including contamination from ambient gasses. In the process of preparing polymerizable compositions, the various ingredients can be exposed to ambient gasses, which are absorbed by the ingredient(s) and contaminate the polymerizable composition. Absorbed gasses such as oxygen interfere with the lens production process by quenching free radicals produced during polymerization. Gasses can also react with the polymer components to produce undesirable by-products. Absorbed gasses also can cause bubbles in the polymerized ophthalmic lens, which severely compromise the optical and mechanical integrity of the lens.
Recent advances in ophthalmic lens compositions and polymerization technologies have dictated corresponding advances in the removal of dissolved contaminants, such as gasses, from the ophthalmic lens composition. Ophthalmic lenses that are being manufactured and developed today are much more sensitive to gaseous contamination than the ophthalmic lenses that were developed just a few years ago. Methods have, accordingly, been developed to remove nearly all dissolved gasses from the polymerizable composition. These methods minimize the risk of solvent volatilization, while maximizing the removal of gasses.
U.S. Pat. No. 5,453,943 to Adams et al., for example, discloses a process that produces a composition having an oxygen concentration of less than 1 part per million. The composition is passed through a selectively porous tubing which, when exposed to a vacuum, draws the gasses from the composition through the tubing. The 943 patent teaches the removal of all dissolved gasses from the composition. Moreover, the 943 patent dictates a strong preference for silicon tubing, apparently because of silicon s strong affinity for oxygen, which allows the tubing to draw oxygen from polymerizable compositions that also have a high affinity for the oxygen. The process of the 943 process suffers, however, from its complexity and time requirements. The polymerizable composition must be drawn through over 60 meters of tubing, and is deoxygenated at a rate of only 8.5 ml./minute.
It was surprising to find, therefore, that quality ophthalmic lenses could be manufactured by a process in which the composition is deoxygenated by bubbling an inert gas such as nitrogen through the composition. Inert gas bubbling surprisingly deoxygenates the polymerizable composition in such a short time period, and under such light flow conditions, that only a minimal quantity of solvent evaporates. Light inert gas bubbling virtually eliminates oxygen content from ophthalmic lens compositions, in less than one twentieth of the time required by the porous tubing process disclosed by U.S. Pat. No. 5,453,943 (see Table 3), even for polymerizable compositions that exhibit a strong affinity for the oxygen.
Moreover, the process is effective even though it merely replaces one dissolved gas, oxygen, with another dissolved gas such as, for example, nitrogen. Lenses produced with compositions that are saturated with inert gas from the bubbling process are statistically indifferent from prior art lenses in which both nitrogen and oxygen are removed.